Electrochemical sensors of the above-noted type necessarily comprise a measurement electrode, a reference electrode and a counterelectrode. Another type of such sensors is also known, which further includes what is called a generator electrode and its counterelectrode. The addition of these two latter electrodes, the effect of which is to modify the concentration of species present in solution, allows the environment of the measurement electrode to be locally controlled.
For example, the pH of the solution may be locally modified by applying a current to the generator electrode. A cathode current will result in the production of OH−ions (the pH then becoming more basic) and, conversely, an anode current will result in the production of H+ ions (the pH then becoming more acid). A counterelectrode associated with the generator electrode, a counterelectrode associated with the measurement electrode (or working electrode) and a reference electrode are necessary in order to produce a complete sensor.
The latter electrodes, the dimensions of which need not be microscopic, are well known in the field in question and may be mounted separately. For example, U.S. Pat. No. 5,597,463 describes a sensor of this second type, which is intended to perform a titration and with which the measurement made is of the potentiostatic type.
It will be readily understood that it is particularly advantageous to use, as measurement electrode, electrodes of very small dimensions, not only because this allows the space between the measurement electrode and the generator electrode to be reduced, but also because the effects of the turbulence of the liquid in the cell are thereby minimized. Such electrodes of small dimensions are referred to without distinction in the rest of the description as either “microelectrodes” or “microdisks,” the latter term being due to the fact that the microelectrodes are usually of circular shape.
Document WO 02/095387 describes a structure, shown in FIG. 1, using an electrically conducting substrate 10, advantageously made of doped silicon, the lower face of said substrate being covered with a metallization layer 11. Its upper face is covered with a passivation layer 12 formed from a multilayer comprising two sublayers, one of SiO2 and the other of Si3N4, which multilayer is known to exhibit excellent stability in an aqueous medium.
The passivation layer 12 is pierced by a regular array of circular through-apertures housing a conducting microdisk 13 substantially thicker than the layer and extending slightly therebeyond so as to avoid any contact of the solution to be measured with the substrate.
The microdisks are formed from the desired electrode material(s), for example by a multilayer comprising titanium, platinum and gold layers. These layers together constitute the measurement electrode of the system.
Document WO 90/12314 proposes an arrangement of the same type, but in which the substrate is made of an inert material and the microelectrodes are constructed on the material by the deposition of successive layers. The contacts to these electrodes are produced through apertures made in the substrate.